1. Field of the Invention
The present invention relates to a backlight unit for liquid crystal display devices using a light emitting diode (LED) and a light guide plate and, more particularly, to a backlight unit for liquid crystal display devices, which can implement local dimming using an edge light source.
2. Description of the Related Art
According to the recent trend of making an image display device have a slim size and high performance, a liquid crystal display (LCD) device has been widely used in a TV, a monitor, and others. Since a liquid crystal panel cannot emit light by itself, the LCD device needs an LCD backlight unit (hereinafter referred to as ‘a backlight unit’). A cold cathode fluorescent lamp (CCFL) has been used as a light source of the backlight unit since it is cheap and is easily assembled.
However, the backlight unit using the CCFL has some defects. For example, it is difficult to implement partial driving such as local dimming or impulsive driving; the use of mercury creates environmental pollution; response rate is slow. To solve these problems, a backlight unit using, as a light source, LEDs instead of a CCFL was proposed.
In an LCD device, it is possible to regulate the luminance of a light source of a backlight unit in such a way that a liquid crystal panel is divided into a plurality of areas, and the luminance is regulated for the respective divided areas according to their gray levels. This driving manner of the backlight unit is called local dimming. That is, the LEDs on the region of the backlight unit corresponding to the bright portion of the liquid crystal panel may be partially turned on, and the others on the rest of the region of the liquid crystal panel may be turned on in low luminance, or otherwise turned off. The impulsive driving manner is one that the backlight unit is timely synchronized with the liquid crystal panel. According to the impulsive driving manner, a plurality of light source regions, arranged on the upper and lower sides of a board of the backlight unit, is turned on in series.
Generally, a backlight unit is divided into a direct backlight unit (direct illumination type) and an edge backlight unit (side illumination type). The edge backlight unit has a bar-type light source, which is disposed on a side of a liquid crystal panel so as to emit light toward the panel via a light guide plate, whereas the direct type backlight unit has a planar light source, which is positioned under a liquid crystal panel to directly illuminate the liquid crystal panel.
FIG. 1 is a perspective view illustrating a conventional edge type backlight unit using LEDs. The backlight unit 10 includes a light guide plate 11, an LED light source section 15, 17 disposed on opposite sides of the light guide plate, and a reflector sheet 19 underlying the light guide plate 11. The LED light source section 15 and 17 has a printed circuit board (PCB) 17 and a plurality of LEDs 15 arranged on the board 17. Light emitted from the LEDs 15 to the light guide plate 11 travels the light guide plate 11 through total internal reflection and is transferred to a liquid crystal panel through light scattering. Such an edge type backlight unit 10 is advantageously fabricated in a relatively small thickness, which however is not appropriate for a partial driving manner, such as local dimming, or so on.
FIG. 2 is a perspective view illustrating a conventional direct type backlight unit using LEDs. In FIG. 2, the backlight unit 20 includes a PCB 21 and a plurality of LEDs 23 thereon. A diffuser plate 25 for diffusing light is disposed between a liquid crystal panel (not shown) and the LEDs 23. The LEDs 23 directly emit light toward a front face of the liquid crystal panel. Such a direct type backlight unit 20 can implement a partial driving manner, such as local dimming or so on. As a method applicable to the local dimming, there are two methods in which one is to control the respective LEDs 23, turning on/off the same, and the other is to divide the backlight unit into uniform regions (e.g., regions A1, A2, and A3), driving the LEDs for the respective regions. However, in the case of individually driving the respective LEDs 23, a problem arises in high power consumption, high cost for protection against high temperature, complexity of a circuit, or others. In the case of driving for the respective regions, a problem occurs in that local dimming effect is relatively deteriorated due to the difficulty in fine division into regions and also a thickness H of the backlight unit. Particularly, since a sufficient thickness H corresponding to an optical thickness is needed to secure uniformity of light, it is disadvantageous to make the backlight unit (and further the LCD device) thinner.